On 5/28/20 1:36 PM, Alexei Starovoitov wrote:
On Thu, May 28, 2020 at 09:50:43AM -0700, Yonghong Song wrote:
With the latest trunk llvm (llvm 11), I hit a verifier issue for
test_prog subtest test_verif_scale1.
The following simplified example illustrate the issue:
w9 = 0 /* R9_w=inv0 */
r8 = *(u32 *)(r1 + 80) /* __sk_buff->data_end */
r7 = *(u32 *)(r1 + 76) /* __sk_buff->data */
......
w2 = w9 /* R2_w=inv0 */
r6 = r7 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
r6 += r2 /* R6_w=inv(id=0) */
r3 = r6 /* R3_w=inv(id=0) */
r3 += 14 /* R3_w=inv(id=0) */
if r3 > r8 goto end
r5 = *(u32 *)(r6 + 0) /* R6_w=inv(id=0) */
<== error here: R6 invalid mem access 'inv'
...
end:
In real test_verif_scale1 code, "w9 = 0" and "w2 = w9" are in
different basic blocks.
In the above, after "r6 += r2", r6 becomes a scalar, which eventually
caused the memory access error. The correct register state should be
a pkt pointer.
The inprecise register state starts at "w2 = w9".
The 32bit register w9 is 0, in __reg_assign_32_into_64(),
the 64bit reg->smax_value is assigned to be U32_MAX.
The 64bit reg->smin_value is 0 and the 64bit register
itself remains constant based on reg->var_off.
In adjust_ptr_min_max_vals(), the verifier checks for a known constant,
smin_val must be equal to smax_val. Since they are not equal,
the verifier decides r6 is a unknown scalar, which caused later failure.
The llvm10 does not have this issue as it generates different code:
w9 = 0 /* R9_w=inv0 */
r8 = *(u32 *)(r1 + 80) /* __sk_buff->data_end */
r7 = *(u32 *)(r1 + 76) /* __sk_buff->data */
......
r6 = r7 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
r6 += r9 /* R6_w=pkt(id=0,off=0,r=0,imm=0) */
r3 = r6 /* R3_w=pkt(id=0,off=0,r=0,imm=0) */
r3 += 14 /* R3_w=pkt(id=0,off=14,r=0,imm=0) */
if r3 > r8 goto end
...
To fix the issue, if 32bit register is a const 0,
then just assign max vaue 0 to 64bit register smax_value as well.
Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
kernel/bpf/verifier.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8d7ee40e2748..5123ce54695f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1174,6 +1174,9 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
reg->smin_value = 0;
if (reg->s32_max_value > 0)
reg->smax_value = reg->s32_max_value;
+ else if (reg->s32_max_value == 0 && reg->s32_min_value == 0 &&
+ tnum_is_const(reg->var_off))
+ reg->smax_value = 0; /* const 0 */
else
reg->smax_value = U32_MAX;
wouldn't this be a more general fix ?
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 01c7d3634151..83450d5d24ab 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1217,11 +1217,11 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
* but must be positive otherwise set to worse case bounds
* and refine later from tnum.
*/
- if (reg->s32_min_value > 0)
+ if (reg->s32_min_value >= 0)
reg->smin_value = reg->s32_min_value;
else
reg->smin_value = 0;
- if (reg->s32_max_value > 0)
+ if (reg->s32_max_value >= 0)
reg->smax_value = reg->s32_max_value;
I thought this way, but not 100% sure about s32_max_value == 0 means
actually the max_value of 0 or some kind of default value (e.g. from
kzalloc). Hence my conservative approach.
I guess you probably right. Let me double check the code.
else
reg->smax_value = U32_MAX;
